FIGS. 24(a) and 24(b) show a prior art semiconductor laser-pumped, solid state laser described in Japanese Published Patent Application 1-122180. FIG. 24(a) shows the entire structure thereof and FIG. 24(b) shows a cross-section taken along line B--B of FIG. 24(a). In FIGS. 24(a) and 24(b), reference numeral 1 designates a semiconductor laser which emits pumping light. The pumping laser is mounted on a heat sink 102. Numeral 103 designates a linear lens. A solid state laser medium 3 comprising, for example, an yttrium aluminum garnet (YAG) crystal is pumped by the pumping light from the semiconductor laser. Numeral 205 designates a film which selectively reflects light and numeral 106 designates a film which totally transmits light. Numerals 70 and 7 designate a totally reflecting mirror and a partially reflecting mirror, respectively. Numeral 2 designates pumping light and numeral 6 designates laser light. Hereinafter, the light emitted from the semiconductor laser 1 is called pumping light and the light emitted from the solid state laser medium 3 is called laser light.
The divergence of the pumping light 2 emitted from the semiconductor laser 1 is narrowed by the linear lens 103 and is incident on the solid state laser medium 3. Narrowing the divergence angle of the light emitted from the semiconductor laser 1 does not lower the pumping light density which is important to obtain a high laser oscillation efficiency. The selective reflecting film 205 has a reflection selectivity that transmits the pumping light 2 and totally reflects the laser light 6. By constructing the optical path in a zigzag configuration in the solid state laser medium 3 in the resonator space between the totally reflecting mirror 70 and the partially reflecting mirror 7, laser light 6, as shown in FIG. 24(a), is produced.
FIG. 23 shows a schematic diagram of a prior art semiconductor laser-pumped, solid state laser described in Mitsubishi Denki Gihoh, Volume 23, Number 4, 1989, pages 287-290. In FIG. 23, numeral 1 designates a semiconductor laser as a pumping light source, and numeral 2 designates a laser beam emitted from the semiconductor laser 1, called pumping light hereinafter. Numerals 8 and 9 designate lenses. Numeral 3 designates a solid state laser medium pumped by the pumping light. Numeral 6 designates laser light emitted from the solid state laser medium 3. Numeral 7 designates a partially reflecting mirror. A totally reflecting coating 32 and a non-reflecting coating 33 which respectively totally reflect and partially reflect the laser light 6 are disposed on the facets of the solid state laser medium, respectively, and, thereby, a laser resonator is formed between the totally reflecting coating 32 and the partially reflecting mirror 7.
The pumping light 2 emitted from the semiconductor laser 1 is collimated by the lens 8 and is concentrated on the solid state laser medium 3 by the lens 9. The pumping light 2 is absorbed and broadens in the solid state laser medium 3 and, thereby, the solid state laser medium 3 is excited. Part of the energy of the pumping light 2 that is absorbed is output as laser light 6.
The conventional semiconductor laser-pumped, solid state laser constructed as described above has the following drawbacks.
In the semiconductor laser-pumped, solid state laser shown in FIG. 24(a), when the divergence angle of the pumping light is large, the idle pumping light which does not contribute to the laser light increases and the pumping efficiency is reduced. The divergence angle of the pumping light in the solid state laser medium varies significantly depending on the positional relationships between the semiconductor laser, the linear lens, and the solid state laser medium that are difficult to establish stably. In addition, in this semiconductor laser-pumped, solid state laser, the optical axes of the laser light and the pumping light must be accurately aligned on the selective reflecting film. However, as the pumping light increases, the thermal distribution in the solid state laser medium changes and the refractive index distribution rises, so that the position of the optical axis varies and deviates from the location where the pumping light is incident. In addition, the optical loss at the selective reflection film is generally large and the total resonator loss increases as the number of reflections increases. Moreover, it is difficult to obtain high power output by arranging a plurality of semiconductor lasers in parallel.
In the semiconductor laser-pumped, solid state laser device of FIG. 23, the broadening of pumping light 2 in the solid state laser medium 3 is large and it is impossible to substantially reduce the pumping cross-section. Therefore, the threshold of laser oscillation is increased, thereby reducing the energy efficiency of the laser oscillation. In addition, the laser light 6 includes many higher order modes and it is difficult to produce a fundamental mode beam which has a good light collecting property.